Papermaker's demands for high speed and efficiency, flexible manufacturing, stringent quality standards, and environmental compatibility coupled with new developments in on-line process control are driving the development of new sensor technology for the paper machine wet-end. The need for better means for providing wet-end chemistry control is emphasized by recent reports that only 10% of the world's one hundred fifty newsprint paper machines operate at above 88% efficiency and over 60% operate under in the low efficiency range of below 82.5%. (Mardon, J., Chinn, G. P., O'Blenes, G., Robertson, G., Tkacz, A. Pulp and Paper Canada, Vol. 99 No. 5 pp. 43-46. (1998).
William E. Scott addressed problems related to wet-end chemistry control. Principles of Wet End Chemistry. Tappi Press, Atlanta, 1996. p 3. "Deposits and scale usually arise from out-of-control wet end chemistry. Typical examples include chemical additive overdosing, charge imbalances, chemical incompatibility, and the shifting of chemical equilibria. All of these phenomena can lead to the formation of precipitates or colloidal aggregates that produce deposits and scale. While there are numerous approaches to treating the symptoms of deposits, the best approach is to determine what is out of control and fix it."
Although variations in the composition and quantity of dissolved solids can lead to problems throughout the paper mill, it is particularly important at the wet-end of the paper machine where the colloidal chemistry must be tuned for optimal machine performance. It is important to gain knowledge and understanding of the relationship between measurements at both the point of origin (pulp mills, bleaching points) and the point of impact (headbox, press-section of the paper machine).
The measurement and characterization of colloidal particles distributed in a liquid stream is an important function in the control of industrial processes involving heterogeneous mixtures. Examples of such processes include pulping and papermaking, water treatment, brewing and food processing, chemical synthesis, and manufacturing. Although numerous methods are available to characterize colloid size and concentration, methods to measure the amount of different colloids components mixed together or to rapidly evaluate the temperature stability of the colloid suspension are not readily available.
Measurements relating the intensity and angular dependence of scattered or absorbed light to the total concentration or size distribution of colloids are available in numerous forms. Instruments for characterizing the amount of colloidal particles that rely on scattering (nepholometry) and attenuation (turbidimetry) are commercially available in laboratory, hand-held, and on-line instruments. On line turbidimeters relate a ratio of light detected in line and at an angle to a source to a turbidity value in Jackson or NTU units. Silveston, U.S. Pat. No. 4,999,514, taught methods for controlling the intensity of the light source to provide a turbidimeter that operates over a broad range of particle concentrations. Kubisiak and Wilson (U.S. Pat. No. 5,331,177) describe an analog to digital turbitimeter apparatus that provides a measure of the change in turbidity over time. Other, more sophisticated, methods involving the analysis of the time and spatial dependence of light attenuation and scattering may provide information on particle size distributions as taught by Strickland et, (U.S. Pat. No. 5,576,827 and the patents referenced therein). Instrumentation specifically designed for measuring particle and fiber size distributions in low consistency (&lt;1%) pulp suspensions by analysis of the time and spatial variation of scattered or absorbed light includes the BTG (British Technology Group) RET-5300 Retention Monitoring System. This instrument employs methods taught by Lundqvist, Pettersson, and Fladda, U.S. Pat. No. 4,318,180. The available instruments do not have a means to differentiate concentrations of similarly sized colloidal pitch particles from colloidal clay particles.
In the area of pulp and paper manufacture, the maintenance of a level of stability and removal of colloidal pitch is an important objective in the wet-end chemistry programs. Deposition leading to poor paper machine efficiency is a costly problem that is addressed through numerous strategies involving pulp processing or chemical addition. Polymer, clay, and talc additives are used to prevent pitch accumulation that may lead to deposition and fouling of pulp processing and papermaking equipment. For example, Cutts taught one method for controlling pitch using micro-particle bentonite addition with cationic polymer flocculation, U.S. Pat. No. 5,676,796. Another combination of using kaolin as inorganic colloid and poly(diallyldimethyl-ammonium chloride) cationic polymer has been taught by Lamar, Pratt, Weber, and Roeder (U.S. Pat. No. 4,964,955). Alternatively, Dreisbach and Barton taught (U.S. Pat. No. 5,266,166) a method of preventing pitch deposits by the addition of nonionic polymeric dispersing agent. A physical process for reducing wood resin pitch from wood process water employing a centrifuge has been taught by Allen and Lapointe (U.S. Pat. No. 5,468,396). The teachings of this invention review the sources and problems associated with wood resin in paper mills and provide further information on physical methods of reducing pitch in pulp and paper process waters.
Chemical and physical methods of controlling pitch may be monitored by turbidity or Zeta, or streaming potential, or charge measurements. Although surface charge or total charge are important measures of the colloidal stability, these measurements do not distinguish colloidal pitch from other, less problematic colloids, such as added clay. Furthermore, turbidity may provide a means of evaluating the total amount of colloidal substance, but pitch colloids are not normally distinguished from other colloids in a turbidity measurement. Typically, no instrumental means are employed to supervise chemical methods of controlling pitch. Despite the high cost of chemical treatment programs and the potential downtime caused by over or under dosing, chemical methods of controlling pitch are often invariant over time and substantial swings in wet-end chemistry.
There is no colloidal pitch measurement available on-line. The accepted laboratory method of pitch analysis using microscopy was taught by Allen (Allen, L. H., Trans. Tech Sect. CPPA 3(2):32 (1977)). This procedure is time consuming, as it has not yet been successfully automated by computerized image analysis techniques. An instrumental method employing a laser beam to count particles flowing through a capillary has been described by Eisenlauer, Horn, Ditter, Eipel, (U.S. Pat. No. 4,752,131). The laser method, known as a pitch counter, requires expensive and specialized instrumentation that is not easily adopted to analysis in an industrial setting.
It is an object of the invention to provide a method of identifying and measuring a characteristic of a colloidal mixture.
It is a further object of this invention to provide a method and means for the rapid determination of an amount of colloidal pitch.
Polychromatic light passed through a colloid sample and detected at an array of wavelengths is a complicated function of the light absorption of the liquid, the light absorption of the particles, the light emission by fluorescence from dissolved or colloidal components, and the scattering that may deflect light away from or towards the detector. The scattering of particles in the range of 0.1-10 times the wavelength of light (Mie scattering) is a complicated function of wavelength, particle size, concentration, and refractive index of the particles, and of the medium. Hence, when the absorption of the colloidal particles is examined, we are looking at both scattering and the UV absorbance of the particles. The wavelength dependence will be a function of the color of the particles, and also the size of the particles. Mie-scattering theory may be solved to back out the particle size distribution from the wavelength dependence of absorption or scattering for a pure or well-characterized substance. But the theory and the calculations are complex, and they are certainly can not be directly applied to analyzing heterogeneous industrial or pulp and paper colloids. The measurements described in this invention provide a means to empirically identify and measure a property related to the particle size, composition, and concentration of a colloidal mixture.